This invention concerns the processing of television signals or video data to obtain interlaced signals in a way which avoids the formation of incorrectly interlaced output.
Television represents moving images by reproducing a sequence of “still” images, each of which represents a temporal sample of an original moving image. Many television systems use “interlaced” scanning formats in which bandwidth is saved by dividing the scanning lines which make up a complete image into (usually two) cyclically repeating groups, and sending only one of these groups for each temporal sample. The full vertical resolution of the image is thus not achieved until a number of temporal samples have been displayed; and, the full temporal resolution is only available at a lower vertical resolution.
The widely-used 525- and 625-line television systems use an interlace factor of two so that two fields of half the total number of lines make up a frame (alternatively known as a picture).
More recently a number of television formats (especially high-definition formats) have been designed which do not use interlace; the full number of image lines is provided at each temporal sample. However, some transmission systems for these formats divide the image into two fields to improve compatibility with equipment designed for interlaced formats.
The relevant system standards define the order in which the two fields of a frame are transmitted. Unfortunately not all systems use the same order; sometimes the topmost line of the picture is in the first field, sometimes it is in the second. This choice is known as field dominance, and there are a number of situations in which it is critical that it is correctly observed.
Modern image processing techniques, compression for example, may combine information from more than one line of the picture, and it is not unusual to combine lines from different fields of the same frame. These processes will fail, or be degraded, if the field dominance is wrong.
There are several sources of field dominance errors. The simplest is where a cut from one image sequence to another occurs between the two fields of the same frame; in this case only half the lines will be available for the last image of the first sequence and for the first image of the second sequence. More complex problems can be caused when the temporal sampling rate of the images differs from that of the transmission system, for example where the speed of action is deliberately changed for effect.
For these reasons it is widely recognised that a method of correcting field dominance errors in a television signal is required. Known methods involve manually editing a recorded version of the programme. Usually cuts are located and incorrectly interlaced frames are removed. Alternatively a defective frame may be replaced by a repeat of a complete existing frame. Obviously deletion of frames changes the length of the programme, and may necessitate changes to the accompanying soundtrack.
These manual repair techniques are very time consuming and an automated process, especially one combined with a format conversion process offers many advantages.
Several methods of automatically detecting cuts are known (for example U.S. Pat. No. 6,075,818) and these methods may be utilised to prevent combining information across a cut in processes such as standards conversion, composite decoding, compression, image manipulation (e.g. warping) and image stabilisation. Noise reduction processes can be improved by using knowledge of cuts as disclosed in U.S. Pat. No. 6,259,489.